Cone beam computed tomography (CBCT) or cone beam CT technology offers considerable promise for providing 3-D volume images. Cone beam CT systems capture volume data sets by using a high frame rate flat panel digital radiography (DR) detector and an x-ray source, typically affixed to a gantry that revolves about the object to be imaged, directing, from various points along its orbit around the subject, a divergent cone beam of x-rays toward the subject. The CBCT system captures projection images throughout the source-detector orbit, for example, with one 2-D projection image at every degree of rotation. The projections are then reconstructed into a 3-D volume image using various techniques. Among the most common methods for reconstructing the 3-D volume image are filtered back projection approaches.
Referring to the perspective view of FIG. 1, there is shown, in schematic form and using enlarged distances for clarity of description, the activity of a conventional CBCT imaging apparatus for obtaining the individual 2-D images that are used to form a 3-D volume image. A cone-beam radiation source 22 directs a cone of radiation toward a subject 20, such as a patient or other subject. A sequence of images is obtained in rapid succession at varying angles about the subject, such as one image at each 1-degree angle increment in a 200-degree orbit. A DR detector 24 is moved to different imaging positions about subject 20 in concert with corresponding movement of radiation source 22. FIG. 1 shows a representative sampling of DR detector 24 positions to illustrate how these images are obtained relative to the position of subject 20. Once the needed 2-D projection images are captured in this sequence, a suitable imaging algorithm, such as filtered back projection or other conventional reconstruction technique, is used for generating the 3-D volume image. Image acquisition and program execution are performed by a computer 30 or by a networked group of computers 30 that are in image data communication with DR detector 24. Image processing and storage is performed using a computer-accessible memory 32. The 3-D volume image can be presented on a display 34.
Although 3-D images of diagnostic quality can be generated using CBCT systems and technology, a number of technical challenges remain. A range of various types of tissue can be of interest to the viewing practitioner, for example. Bone and soft tissue have different characteristics for radiographic imaging and often show improved levels of detail under different processing conditions. Thus, for example, imaging filters that are optimized for bone imaging can perform poorly in providing diagnostically useful images of soft tissue. Similarly, imaging filters and processing that work well in showing image details in soft tissue may exhibit disappointing results if used for processing bone content.
Another difficulty with display of detail and obtaining good contrast, brightness, sharpness, and other characteristics relates to the inherent limitations of the human eye and of the display hardware. The image data itself can be 12-bit data, capable of representing 4,096 grayscale shades. The display hardware is much more limited and typically has an 8-bit grayscale range, capable of representing 256 shades of gray. The practitioner or other viewer, however, can differentiate no more than about 90 different shades of gray at best; some viewers are able to discern fewer than 50 different grayscale tones from each other.
Conventional display systems typically accommodate this disparity in grayscale mapping by providing a windowing adjustment for radiographic images. Windowing maps a partial portion of the larger image grayscale range to the smaller, limited dynamic range of the display hardware. Values above or below the mapped range are clipped. This means that the same brightness is assigned to all pixels having a value that exceeds the maximum brightness range; similarly, the same dark value is assigned to all pixels outside the range in the other direction. The practitioner or other viewer has a single brightness control that adjusts the window mapping toward one end of the image grayscale range or the other.
Many practitioners who are familiar with windowing accept its limitations and struggle with the available windowing adjustments to obtain the detail they need for diagnostic purposes. Adjustment of the window to one setting may provide good detail for bone and skeletal structures; however, image detail contrast for nearby soft tissue is degraded. Changing the window adjustment may improve the visibility of soft tissue detail but can then compromise bone contrast and detail. Windowing is thus often unsatisfactory where it is desirable to view both bone and soft tissue structure to a high level of detail in a reconstructed image.
To address this problem, commonly assigned U.S. Patent Application No. 2013/0004041 by Yang et al. proposes fusion of bone and soft tissue data from the 2-D projection images that were obtained using the process shown in FIG. 1, then generating a volume image resulting from this fusion. While this method has merit, however, there can be drawbacks to generating a single volume reconstruction using multiple sets of projection data. Processing time for generating the final “fused” 3-D volume reconstruction can increase significantly, without compensating improvements in the capability to adjust tone scale for image slice display. Improved techniques for enhancing the display of such fused data are still needed. Window level adjustment, although it offers some capability to change the appearance of some image features in local areas of the image, does not adequately address the need for enhancement of overall image content. For example, tone scale adjustment is not provided for the different features in the fused 3-D volume. Only windowing adjustment is provided for the displayed output, limiting the amount of adjustment available to the viewing practitioner.
Thus, it is seen that there would be value in an imaging approach that provides a high level of imaging detail for soft tissue as well as for bone or other anatomical features and that provides improved adjustments for the displayed image content.